1. Field of the Invention
The present invention relates to a liquid crystal driving electrode, and a liquid crystal display device (LCD), and a manufacturing method thereof. More specifically, the present invention relates to an active-matrix LCD (AM-LCD) applied to the in-plane switching (IPS) mode and a manufacturing method thereof.
2. Description of the Prior Art
LCDs are generally characterized by thin profile, light weight, and low power consumption. In particular, AM-LCDs, which drive individual pixels arranged in a horizontal and vertical matrix by use of active elements, are regarded as high-quality flat panel display devices. Among them, thin film transistor LCDs (TFT-LCDs) applying thin film transistors (TFTs) as the active elements for switching the individual pixels are widely used.
A conventional AM-LCD utilizes a twisted nematic (TN) type electro-optical effect. Moreover, liquid crystal molecules are sandwiched between two substrates and are activated by applying an electric field that is substantially perpendicular to substrate surfaces thereto. Meanwhile, a method applying comb-shaped electrodes as driving electrodes has been disclosed in U.S. Pat. No. 3,807,831 (hereinafter referred to as Patent Document 1) as a technique for an IPS-LCD which activates liquid crystal molecules by an electric field being substantially parallel to substrate surfaces.
Moreover, Japanese Unexamined Patent Publication No. S56 (1981)-091277 (hereinafter referred to as Patent Document 2) discloses another technique applying comb-shaped electrodes that are engaged with each other like the foregoing case. The object of this technique is to reduce parasitic capacitance between a common electrode and a drain bus line or between the common electrode and a gate bus line in an AM-LCD applying the TN type electro-optical effect.
Japanese Unexamined Patent Publication No. H07 (1995)-036058 (hereinafter referred to as Patent Document 3) disclosed a technique to apply the IPS mode to a TFT-LCD. Moreover, Japanese Unexamined Patent Publication No. H10 (1998)-307295 (hereinafter referred to as Patent Document 4) disclosed a technique which bends an electrode generating a horizontal electric field. In Patent Document 4, when applying an electric field, a driving direction (a turning direction) of liquid crystal molecules is changed depending on the region by use of these bent portions, thereby reducing display coloring at an oblique view angle.
FIG. 1A and FIG. 1B show an example of a configuration of the IPS-LCD disclosed in Patent Document 4. FIG. 2 is a view for explaining electric lines of force generated by driving electrodes shown in FIG. 1A. As shown in FIG. 1A, this LCD includes each pixel region in a region surrounded by multiple gate bus lines 55 extending in a horizontal direction and multiple drain bus lines 56 extending substantially in a vertical direction in a bent manner. Such pixel regions are arranged in a horizontal and vertical matrix to form a display area as a whole. An active element 54 is a TFT made of amorphous silicon (hereinafter abbreviated as a-Si). Moreover, the each active element 54 is formed in the vicinity of the crossing of the gate bus line 55 and the drain bus line 56 for each pixel. Meanwhile, since the drain bus lines 56 extend in the vertical direction in the bent manner in FIG. 1A, the shape of the pixel constituting the horizontal and vertical matrix is bent into a V shape.
Pixel electrodes 71D and common electrodes 72D for generating an electric field are formed into a laterally-facing ladder shape (a comb shape) as illustrated in FIG. 1A. The pixel electrodes 71D and the common electrodes 72D are alternately located in the steps of the ladder. To be more precise, the respective steps of the ladder are bent into the V shape in the respective pixel regions to be aligned with the drain bus lines 56. At the bent position, the pixel region is divided into a sub region 1 on an upper side and a sub region 2 on a lower side in the drawing. The direction of inclination of the electrode caused by bending in the V shape is shifted clockwise in the vertical direction of the drawing in terms of the sub region 1. On the contrary, the direction of inclination of the electrode caused by bending in the V shape is shifted counterclockwise in the vertical direction of the drawing in terms of the sub region 2. Each of the pixel electrodes 71D and the common electrodes 72D partially overlap each other with an interlayer insulating film 57 disposed therebetween (see FIG. 1B). Such an overlapping portion constitutes an additional capacitance. To avoid disconnection of the common electrode 72D, it is formed to stride over the adjacent pixel in the direction of extension of the gate bus line 55 by using two lines of a B line and a C line located on the upper side and the lower side of FIG. 1A.
As shown in FIG. 1B, the common electrodes 72D, the pixel electrodes 71D, and the drain bus lines 56 are formed on a first substrate 11. The common electrodes 72D are insulated from the pixel electrodes 71D and the drain bus lines 56 by use of the interlayer insulating film 57. Although it is not illustrated in FIG. 1B, the gate bus lines 55, like the common electrodes 72D, are also insulated from the pixel electrodes 71D and the drain bus lines 56 by use of the interlayer insulating film 57. These structures formed on the substrate 11 are covered with a passivation film 59. An alignment film 31 made of an organic polymer film is formed on a surface of an active-matrix substrate including these constituents, and a surface thereof is subjected to an aligning treatment.
Meanwhile, color filters (not shown) including the three primary colors of red, green, and blue are provided on a second substrate 12 constituting a counter substrate to the active-matrix substrate to correspond to the respective pixel regions, and a light-shielding black matrix (not shown) is provided in the region other than the regions corresponding to the respective pixel regions. In addition, an alignment film 32 made of an organic polymer film is formed on a surface thereof, and a surface of the alignment film 32 is subjected to an aligning treatment.
The active-matrix substrate is put on the counter substrate to keep a certain interval while the surfaces having the alignment films 31 and 32 being set inside, and a liquid crystal layer 20 is inserted between the both substrates. Moreover, a pair of polarizing plates (not shown) are disposed outside the both substrates.
As shown in FIG. 1A, the surfaces of the alignment films 31 and 32 are uniformly subjected to the aligning treatment such that liquid crystal molecules 21 are aligned in parallel to the longitudinal direction of the drawing (the vertical direction in the drawing) when no electric field is applied. Directions of transmission axes of this pair of polarizing plates are set perpendicular to each other and the transmission axis of one of the polarizing plates coincides with the initial aligning direction (the aligning direction at the time of no electric field) of the liquid crystal which is uniformly subjected to the aligning treatment.
Next, a manufacturing process of the liquid crystal display device of FIG. 1A will be described. First, the gate bus lines 55 and the common electrodes 72D made of chromium (Cr) are formed on the first substrate 11 such as a glass substrate, and the interlayer insulating film 57 made of silicon nitride (SiNx) is formed to cover these constituents. Subsequently, a-Si film serving as an active layer of transistors is formed into island shapes on the gate bus lines 55 with the interlayer insulating film 57 interposed in therebetween. Further, the drain bus lines 56 and the pixel electrodes 71D made of Cr are formed thereon. Next, the passivation film 59 made of SiNx is formed to cover these structures. The color filters and the light-shielding black matrix are formed on the second substrate 12 such as a glass substrate.
As described above, alignment films made of polyimide are formed on the respective surfaces of the active-matrix substrate and the color filter substrate constructed. The alignment films are uniformly subjected to the aligning treatment. Thereafter, the both substrates are put together to keep an interval of 4.5 μm, for example. Then, for example, nematic liquid crystal having refractive index anisotropy of 0.067, is filled between the substrates within a vacuum chamber. Thereafter, the polarizing plates are attached to the outer surfaces of the both substrates.
In the configuration shown in FIG. 1A, a liquid crystal driving electric field upon voltage application is generated in the direction slightly inclined clockwise relative to the lateral direction of the drawing in terms of the sub region 1 and is generated in the direction slightly inclined counterclockwise relative to the lateral direction of the drawing in terms of the sub region 2. Accordingly, at the time of no electric field, liquid crystal molecules 21 that are uniformly aligned along the longitudinal direction in the drawing (the vertical direction in the drawing) are turned counterclockwise in the sub region 1 and clockwise in the sub region 2, respectively.
As described above, the turning directions of the liquid crystal molecules are different between the two sub regions 1 and 2. In this way, it is possible to suppress display coloring attributable to a change in the view angle.
FIG. 3A is a plan view for explaining another example of the configuration of the IPS-LCD disclosed in Patent Document 4. Meanwhile, FIG. 3B is a view for explaining electric lines of force generated by electrodes in FIG. 3A. The configuration in FIG. 3A is common to the configuration in FIG. 1A in many aspects. However, pixel electrodes 71E and common electrodes 72E have protrusions extending along boundaries of sub regions at bent portions. In the configuration shown in FIG. 3A, it is possible to avoid reverse turns of the liquid crystal molecules relative to desired turning directions in the respective sub regions even in the vicinity of the regions where the electrodes are bent into the V shape. Accordingly, it is possible to achieve uniform and stable display.
FIG. 4A is a plan view for explaining still another example of the configuration of the IPS-LCD which is disclosed in Japanese Unexamined Patent Publication No. 2001-305567 (hereinafter referred to as Patent Document 5) and FIG. 4B is a plan view showing driving electrodes thereof. In this LCD, pixel electrodes and common electrodes are adjacently provided on one of substrates. The intervals between these two electrodes are equal to 5 μm or more when the electrodes are viewed from a parallel direction to the surface of the substrate toward the direction of extension of signal lines. The configuration in FIG. 4A shows the example of this LCD. As shown in the drawing, mutually opposed surfaces of a pixel electrode 71F and a common electrode 72F have curved portions.
When there are the intervals of 5 μm or more between the pixel electrodes 71F and the common electrodes 72F, a rubbing cloth can enter groove portions between the pixel electrodes 71F and the common electrodes 72F favorably. Accordingly, spaces between the electrodes are uniformly subjected to a rubbing treatment whereby liquid crystal molecules 21 seem to be uniformly aligned in the direction of signal lines 3. Moreover, since the mutually opposed surfaces of the pixel electrodes 71F and the common electrodes 72F have the curved portions, the directions of the electric lines of force vary depending on the location of the curved portions. For this reason, the directions of the liquid crystal molecules 21 also vary upon application of an arbitrary voltage, and a view angle characteristic in an oblique direction is thereby improved.
Nevertheless, the above-described IPS-LCDs of the prior art still have unsolved problems. Specifically, disturbance in liquid crystal domains is apt to occur in the configuration shown in FIG. 1A due to the lack of a structure for stabilizing the boundary portions of the sub regions. In particular, if the size of each domain is minimized for the purpose of achieving higher definition of pixels, the disturbance in the liquid crystal domains is easy to be occurred as adjacent liquid crystal domains may be irregularly fused and necessary liquid domains may disappear. In such a case, the display may become rough or blocky. Such a problem may be notable particularly when a panel is pressed with a finger. When the disturbance in the liquid crystal domains is caused by finger pressing or the like, it is necessary to stop a device (cut off a power source) and leave the device for a while in order to recover the liquid crystal domains.
In the IPS-LCD in which the electrodes for generating the lateral electric field are bended into the V shape, the turning directions of the liquid crystal molecules in the respective sub regions are more clearly defined due to a relation between the direction of the electric field and the initial aligning direction of the liquid crystal as the bent angle becomes greater. Accordingly, the turning directions of the liquid crystal molecules at the boundary portions are stabilized. Meanwhile, in view of a voltage transmission rate characteristic, it is preferable to set the angle between the direction of extension of the electrodes and the initial aligning direction of the liquid crystal approximately in a range of 5 to 25 degrees. This is because it is necessary to turn the direction of liquid crystal alignment virtually in the amount of 45 degrees using the electric field for the purpose of switching between a dark (black) state and a bright (white) state. That is, according to the prior art, it is not easy to set an appropriate angle for achieving the favorable voltage transmission rate characteristic while stabilizing the boundaries of the sub regions.
Meanwhile, in the configuration shown in FIG. 3A, the protrusions are formed at the bent portions in order to stabilize the liquid crystal domains in the respective sub regions. In this configuration, it is possible to stabilize the liquid crystal domains since the direction of the electric field is regulated in the vicinity of the bent portions. However, a distance between the pixel electrode 71 and the common electrode 72 is reduced in the vicinity of the protrusions of the electrodes. Accordingly, a stronger electric field is generated in that region as compared to the region actually used for the display. When performing a dark (black) display, a voltage below a threshold voltage for causing a change in the alignment of the liquid crystal is usually applied (in the case of a normally black mode). However, the relatively strong electric field in the vicinity of the protrusion still leads to a change in the aligning direction of the liquid crystal molecules in that case, thereby causing light leakage (a black blur) that triggers deterioration in display contrast. In addition, this configuration also has a problem that a short circuit between electrodes is apt to occur in a region in the vicinity of the protrusion of the electrode where a distance between the liquid crystal driving electrodes is short.
Meanwhile, in the configuration shown in FIG. 4A, the mutually opposed surfaces of the pixel electrode 71F and the common electrode 72F have the curved portions so that the directions of the electric lines of force vary depending on the location of the curved portions of the electrodes. For this reason, this configuration bears such a problem that it is not possible to define a uniform angle between the direction of extension of the electrode and the initial aligning direction of the liquid crystal in terms of the voltage transmission rate characteristic in sub region for the purpose of optimization.